Ink jet printing is performed by discharging ink droplets from a print head to a substrate. The droplets are ejected through orifices or nozzles in the print head and are directed to the substrate to form an image thereon. In contrast to many other types of printing, there preferably is no contact between the printer and the substrate with ink jet printing.
Most of the ink jet printers known in the art may be characterized as either continuous or impulse devices, depending upon the mechanism by which the ink droplets are directed to the substrate. In continuous ink jet systems, an essentially uninterrupted stream of ink is ejected from a nozzle and breaks up into droplets. The droplets bear an electric charge so that they can be deflected by an applied electric field which is modulated according to the particular image to be recorded. The electric field directs the droplets toward either the substrate or an ink re-circulating reservoir.
With so-called "impulse" or "drop-on-demand" ink jet printers, image formation is controlled by selectively energizing and de-energizing, for example, a piezoelectric transducer or solenoid rather than by modulating an applied electric field. Ink is stored in the print head or nozzle until it is necessary to form an image on the substrate. The printer is then activated by print signals to apply pressure to the ink and discharge a selected number of discrete ink droplets toward the substrate.
Because ink is ejected from impulse-type printers only periodically, these devices present a number of problems which typically are not encountered in continuous ink jet systems. These problems, which occur during the relatively short intervals between individual print signals during a single print cycle, include irregularly shaped drops and/or improper spacing of drops. The root cause of these problems may be attributable to movement of the ink meniscus at the time a print signal is generated, particularly where efforts are made to print at a frequency in excess of 3 KHz. One approach to these problems is presented by U.S. Pat. No. 4,266,232, in the name of Juliana, Jr., et al., which discloses an impulse printer wherein ink drops of substantially uniform size and spacing are generated by applying drive pulses in a mutually synchronous fashion at every one of predetermined equal intervals. The amplitude of the drive pulses is controlled so that the amplitude of the drive pulse is below that of a print signal when no drop is to be formed. An even better approach is presented by U.S. Pat. No. 4,459,601, in the name of Howkins, wherein a fill-before-fire mode of operation is disclosed, i.e., a pulse of predetermined length is used to initiate filling of the jet chamber and firing of a droplet occurs on the trailing edge of the pulse.
Certain other problems associated with impulse ink jet printers relate to the considerably longer intervals between print cycles. Unlike continuous ink jet printers, impulse devices typically are maintained in stand-by or quiescent modes for relatively long intervals, sometimes on the order of seconds, minutes, and even hours. During these intervals, ink is allowed to stand, thicken due to evaporation of ink components, and possibly clog the nozzles of the print head. Impulse printers may begin a printing cycle with such thickened material in place. Many of the start-up problems encountered with impulse printers are attributable to ink which has been allowed to clog the nozzles during quiescent periods. Ink clogging is less of a concern in continuous systems because there typically are fewer interruptions in the flow of ink and any such interruption is of considerably shorter duration. Even where ink is allowed to stand and solidify in a continuous ink jet printer, it is more easily purged due to the considerably higher pressures at which these devices operate.
A number of methods and apparatus are known in the art for preventing clogging in ink jet printers during quiescent periods. For example, U.S. Pat. No. 4,970,527, in the name of Gatten, discloses an ink jet printer which prevents clogging by printing a few ink dots when the printer is idle. The method of Gatten, however, wastes both ink and printing substrate.
U.S. Pat. No. 3,925,789, in the name of Kashio, discloses an ink jet recording device which comprises a timer for determining the length of a quiescent period and a means for preliminarily ejecting ink from a nozzle if the quiescent period exceeds a predetermined amount of time. The ejected ink is not directed to a printing substrate but, rather, to an ink collector. U.S. Pat. No. 4,540,997, in the names of Biggs, etal., discloses an ink jet printer wherein clogging is minimized by transporting the nozzles during quiescent periods to communicate with a wash station and then ejecting ink from the nozzles into the wash station if the printer has not functioned for a predetermined period of time.
Therefore, there exists a need for relatively simple methods and apparatus for preventing ink jet clogging which do not waste ink or printing substrate and which do not require additional devices such as ink collectors and washing stations.